fix(zero length): Improve rendering for zero-length error spans

A zero-length span at a line boundary can be associated either with the
previous or the next line. Current code prefer to use the next line,
however there isn't always a "next line" (end of source, or "no context"
configuration).
There is also the extra-special case of an empty document which has no
"next line" but also no "previous line".

This commit adds an empty newline at the end of a document if appropriate
so that there is a "next line", or use the "previous line"

src/handlers/graphical.rs

@@ -515,7 +515,19 @@ impl GraphicalReportHandler {
         context: &LabeledSpan,
         labels: &[LabeledSpan],
     ) -> fmt::Result {
-        let (contents, lines) = self.get_lines(source, context.inner())?;
+        let (contents, mut lines) = self.get_lines(source, context.inner())?;
+
+        // If the number of lines doesn't match the content's line_count, it's
+        // because the content is either an empty source or because the last
+        // line finishes with a newline. In this case, add an empty line.
+        if lines.len() != contents.line_count() {
+            lines.push(Line {
+                line_number: contents.line() + contents.line_count(),
+                offset: contents.span().offset() + contents.span().len(),
+                length: 0,
+                text: String::new(),
+            });
+        }
 
         // only consider labels from the context as primary label
         let ctx_labels = labels.iter().filter(|l| {
@@ -570,6 +582,10 @@ impl GraphicalReportHandler {
             self.theme.characters.hbar,
         )?;
 
+        // Save that for later, since `content` might be moved before we need
+        // that info
+        let has_content = !contents.data().is_empty();
+
         // If there is a primary label, then use its span
         // as the reference point for line/column information.
         let primary_contents = match primary_label {
@@ -600,48 +616,53 @@ impl GraphicalReportHandler {
         }
 
         // Now it's time for the fun part--actually rendering everything!
-        for line in &lines {
-            // Line number, appropriately padded.
-            self.write_linum(f, linum_width, line.line_number)?;
-
-            // Then, we need to print the gutter, along with any fly-bys We
-            // have separate gutters depending on whether we're on the actual
-            // line, or on one of the "highlight lines" below it.
-            self.render_line_gutter(f, max_gutter, line, &labels)?;
-
-            // And _now_ we can print out the line text itself!
-            let styled_text =
-                StyledList::from(highlighter_state.highlight_line(&line.text)).to_string();
-            self.render_line_text(f, &styled_text)?;
-
-            // Next, we write all the highlights that apply to this particular line.
-            let (single_line, multi_line): (Vec<_>, Vec<_>) = labels
-                .iter()
-                .filter(|hl| line.span_applies(hl))
-                .partition(|hl| line.span_line_only(hl));
-            if !single_line.is_empty() {
-                // no line number!
-                self.write_no_linum(f, linum_width)?;
-                // gutter _again_
-                self.render_highlight_gutter(
-                    f,
-                    max_gutter,
-                    line,
-                    &labels,
-                    LabelRenderMode::SingleLine,
-                )?;
-                self.render_single_line_highlights(
-                    f,
-                    line,
-                    linum_width,
-                    max_gutter,
-                    &single_line,
-                    &labels,
-                )?;
-            }
-            for hl in multi_line {
-                if hl.label().is_some() && line.span_ends(hl) && !line.span_starts(hl) {
-                    self.render_multi_line_end(f, &labels, max_gutter, linum_width, line, hl)?;
+        // (but only if we have content or if we wanted content, to avoid
+        // pointless detailed rendering, e.g. arrows pointing to nothing in the
+        // middle of nothing)
+        if has_content || self.context_lines.is_some() {
+            for (line_no, line) in lines.iter().enumerate() {
+                // Line number, appropriately padded.
+                self.write_linum(f, linum_width, line.line_number)?;
+
+                // Then, we need to print the gutter, along with any fly-bys We
+                // have separate gutters depending on whether we're on the actual
+                // line, or on one of the "highlight lines" below it.
+                self.render_line_gutter(f, max_gutter, line, &labels)?;
+
+                // And _now_ we can print out the line text itself!
+                let styled_text =
+                    StyledList::from(highlighter_state.highlight_line(&line.text)).to_string();
+                self.render_line_text(f, &styled_text)?;
+
+                // Next, we write all the highlights that apply to this particular line.
+                let (single_line, multi_line): (Vec<_>, Vec<_>) = labels
+                    .iter()
+                    .filter(|hl| line.span_applies(hl, line_no == (lines.len() - 1)))
+                    .partition(|hl| line.span_line_only(hl));
+                if !single_line.is_empty() {
+                    // no line number!
+                    self.write_no_linum(f, linum_width)?;
+                    // gutter _again_
+                    self.render_highlight_gutter(
+                        f,
+                        max_gutter,
+                        line,
+                        &labels,
+                        LabelRenderMode::SingleLine,
+                    )?;
+                    self.render_single_line_highlights(
+                        f,
+                        line,
+                        linum_width,
+                        max_gutter,
+                        &single_line,
+                        &labels,
+                    )?;
+                }
+                for hl in multi_line {
+                    if hl.label().is_some() && line.span_ends(hl) && !line.span_starts(hl) {
+                        self.render_multi_line_end(f, &labels, max_gutter, linum_width, line, hl)?;
+                    }
                 }
             }
         }
@@ -1271,26 +1292,38 @@ impl Line {
 
     /// Returns whether `span` should be visible on this line, either in the gutter or under the
     /// text on this line
-    fn span_applies(&self, span: &FancySpan) -> bool {
+    ///
+    /// An empty span at a line boundary will preferable apply to the start of
+    /// a line (i.e. the second/next line) rather than the end of one (i.e. the
+    /// first/previous line). However if there are no "second" line, the span
+    /// can only apply the "first". The `inclusive` parameter is there to
+    /// indicate that `self` is the last line, i.e. that there are no "second"
+    /// line.
+    fn span_applies(&self, span: &FancySpan, inclusive: bool) -> bool {
         // A span applies if its start is strictly before the line's end,
         // i.e. the span is not after the line, and its end is strictly after
         // the line's start, i.e. the span is not before the line.
         //
-        // One corner case: if the span length is 0, then the span also applies
-        // if its end is *at* the line's start, not just strictly after.
-        (span.offset() < self.offset + self.length)
-            && match span.len() == 0 {
-                true => (span.offset() + span.len()) >= self.offset,
-                false => (span.offset() + span.len()) > self.offset,
-            }
+        // Two corner cases:
+        // - if `inclusive` is true, then the span also applies if its start is
+        //   *at* the line's end, not just strictly before.
+        // - if the span length is 0, then the span also applies if its end is
+        //   *at* the line's start, not just strictly after.
+        (match inclusive {
+            true => span.offset() <= self.offset + self.length,
+            false => span.offset() < self.offset + self.length,
+        }) && match span.len() == 0 {
+            true => (span.offset() + span.len()) >= self.offset,
+            false => (span.offset() + span.len()) > self.offset,
+        }
     }
 
     /// Returns whether `span` should be visible on this line in the gutter (so this excludes spans
     /// that are only visible on this line and do not span multiple lines)
     fn span_applies_gutter(&self, span: &FancySpan) -> bool {
         // The span must covers this line and at least one of its ends must be
         // on another line
-        self.span_applies(span)
+        self.span_applies(span, false)
             && ((span.offset() < self.offset)
                 || ((span.offset() + span.len()) >= (self.offset + self.length)))
     }

src/source_impls.rs

@@ -20,6 +20,16 @@ fn context_info<'a>(
 ) -> Result<MietteSpanContents<'a>, MietteError> {
     let mut iter = input
         .split_inclusive(|b| *b == b'\n')
+        .chain(
+            // `split_inclusive()` does not generate a line if the input is
+            // empty or for the "last line" if it terminates with a new line.
+            // This `chain` fixes that.
+            match input.last() {
+                None => Some(&input[0..0]),
+                Some(b'\n') => Some(&input[input.len()..input.len()]),
+                _ => None,
+            },
+        )
         .enumerate()
         .map(|(line_no, line)| {
             // SAFETY:
@@ -291,8 +301,8 @@ mod tests {
         let contents = src.read_span(&(12, 0).into(), None, None)?;
         assert_eq!("", std::str::from_utf8(contents.data()).unwrap());
         assert_eq!(SourceSpan::from((12, 0)), *contents.span());
-        assert_eq!(2, contents.line());
-        assert_eq!(4, contents.column());
+        assert_eq!(3, contents.line());
+        assert_eq!(0, contents.column());
         assert_eq!(1, contents.line_count());
         Ok(())
     }

tests/graphical.rs

@@ -288,6 +288,9 @@ fn empty_source() -> Result<(), MietteError> {
 
   × oops!
    ╭─[bad_file.rs:1:1]
+ 1 │ 
+   · ▲
+   · ╰── this bit here
    ╰────
   help: try doing it better next time?
 "#
@@ -633,6 +636,78 @@ fn single_line_highlight_offset_end_of_line() -> Result<(), MietteError> {
     Ok(())
 }
 
+#[test]
+fn single_line_highlight_offset_end_of_file_no_newline() -> Result<(), MietteError> {
+    #[derive(Debug, Diagnostic, Error)]
+    #[error("oops!")]
+    #[diagnostic(code(oops::my::bad), help("try doing it better next time?"))]
+    struct MyBad {
+        #[source_code]
+        src: NamedSource<String>,
+        #[label("this bit here")]
+        highlight: SourceSpan,
+    }
+
+    let src = "one\ntwo\nthree".to_string();
+    let err = MyBad {
+        src: NamedSource::new("bad_file.rs", src),
+        highlight: (13, 0).into(),
+    };
+    let out = fmt_report(err.into());
+    println!("Error: {}", out);
+    let expected = r#"oops::my::bad
+
+  × oops!
+   ╭─[bad_file.rs:3:6]
+ 2 │ two
+ 3 │ three
+   ·      ▲
+   ·      ╰── this bit here
+   ╰────
+  help: try doing it better next time?
+"#
+    .trim_start()
+    .to_string();
+    assert_eq!(expected, out);
+    Ok(())
+}
+
+#[test]
+fn single_line_highlight_offset_end_of_file_with_newline() -> Result<(), MietteError> {
+    #[derive(Debug, Diagnostic, Error)]
+    #[error("oops!")]
+    #[diagnostic(code(oops::my::bad), help("try doing it better next time?"))]
+    struct MyBad {
+        #[source_code]
+        src: NamedSource<String>,
+        #[label("this bit here")]
+        highlight: SourceSpan,
+    }
+
+    let src = "one\ntwo\nthree\n".to_string();
+    let err = MyBad {
+        src: NamedSource::new("bad_file.rs", src),
+        highlight: (14, 0).into(),
+    };
+    let out = fmt_report(err.into());
+    println!("Error: {}", out);
+    let expected = r#"oops::my::bad
+
+  × oops!
+   ╭─[bad_file.rs:4:1]
+ 3 │ three
+ 4 │ 
+   · ▲
+   · ╰── this bit here
+   ╰────
+  help: try doing it better next time?
+"#
+    .trim_start()
+    .to_string();
+    assert_eq!(expected, out);
+    Ok(())
+}
+
 #[test]
 fn single_line_highlight_include_end_of_line() -> Result<(), MietteError> {
     #[derive(Debug, Diagnostic, Error)]
@@ -1088,9 +1163,8 @@ fn multiline_highlight_flyby() -> Result<(), MietteError> {
 line2
 line3
 line4
-line5
-"#
-    .to_string();
+line5"#
+        .to_string();
     let len = src.len();
     let err = MyBad {
         src: NamedSource::new("bad_file.rs", src),
@@ -1147,9 +1221,8 @@ fn multiline_highlight_no_label() -> Result<(), MietteError> {
 line2
 line3
 line4
-line5
-"#
-    .to_string();
+line5"#
+        .to_string();
     let len = src.len();
     let err = MyBad {
         source: Inner(InnerInner),
@@ -2267,6 +2340,8 @@ fn multi_adjacent_zero_length_no_context() -> Result<(), MietteError> {
    · ▲
    · ╰── this bit here
  3 │ thr
+   ·    ▲
+   ·    ╰── and here
    ╰────
   help: try doing it better next time?
 "#